Experiments were conducted on the calcareous red alga, Corallina elongata, a species representative of shallow water vegetal cover in Mediterranean areas with biomass ranging from 820 to 2544 gDW.m(-2), in order to estimate its productivity and calcification rates. Carbonate and oxygen budgets were estimated on samples incubated in situ under natural light cycles, by measuring initial and final dissolved oxygen, pH and alkalinity levels. In light conditions, oxygen concentrations and pH values increased as a consequence of oxygen production and carbon dioxide consumption due to the productivity process, and were a direct function of sample biomass. Strictly-reverse dynamics were recorded in dark conditions. A comparison of photosynthetic performances was conducted on a non-calcareous green alga, Ulva rigida, which showed higher rates of oxygen production and pH modification than C. elongata, but no significant change in total alkalinity. For C. elongata, a significant decrease in total alkalinity with incubation time was observed under light conditions, which was directly related to the algal sample biomass (R-2= 0.95; n= 16). Light to dark calcification ratio (L/D) was about 3.6. In these experiments, the photosynthetic quotient of C. elongata was 0.89, its net carbon productivity was 2.5 g C.m(-2).d(-1), gross production to daily respiration (Pg/R) was about 4.9 and its calcification rate was estimated at 13.8 g CaCO3 M-2 d(-1).